Regulation concerning power factor correction for a wide range of devices is becoming increasingly stringent. For example, new regulation requires power factor correction, for any light emitting diode (LED) power supply with output power higher than 5 W.
For low to medium power level (e.g., 5 W to 100 W), a flyback converter is often used. FIG. 1 shows a block diagram of such a system. By forcing the average input current to follow the input voltage, high power factor can be achieved. In order to reduce the cost, critical conduction mode is often used to achieve power factor correction. However, this results in a ripple in the output voltage at harmonics of the line frequency. The second harmonic (e.g., 120 Hz for North America or 100 Hz for China, Europe) is of particular concern for DC lighting applications, such as LED lighting, as it results in visible flickering wherein the human eye can see fluctuation of the light emitting from the LED.
In such a conventional design there is a compromise between power factor and small low frequency current ripple through the load. For example, in order to achieve a power factor higher than 90%, the peak to peak value of the load current ripple can be as high as 60% of the average DC value. For example, for an average load current of 0.5 A, the low frequency current ripple can be as high as 0.3 A (peak to peak). This raises several problems: Firstly, as mentioned above, for DC lighting (e.g., LED) applications, the ripple current causes visible flickering. Secondly, it is difficult to achieve variable output power. When the average load current is reduced, the ripple current does not decrease proportionally and therefore the ripple current will become more of a problem at reduced output power. In DC lighting applications, flickering will be worse at reduced brightness. Thirdly, the ripple current degrades the lifespan of many devices, such as LEDs.
To achieve high power factor and small low frequency current ripple, two-stage conversion may be used. FIG. 2 shows a circuit block diagram of a conventional converter used to drive an LED, where the first stage 20 is typically a boost converter that converts the AC voltage into a high voltage, e.g., 400 V, and also achieves power factor correction. The second stage 22 is a DC to DC converter that converts the 400 V into a lower voltage required by the load 100, e.g., 50 V or 125 V, provides electrical isolation, and regulates the load current. However, compared to the converter of FIG. 1, the converter of FIG. 2 suffers from the drawbacks of higher cost and larger size.